Compact directional coupler



Jan. 18, 1966 5, COHN 3,230,482

COMPACT DIRECTIONAL COUPLER Filed Oct. 28, 1963 FIG.I

nel

Hil A CUJQQQQ INVENTOR,

SEYMOUR B. COHN ATTOR N EY.

United States Patent 3,230,482 COMPACT DIRECTIONAL COUPLER Seymour B.Cohn, Calabasas, Califi, assignor to the United States of America asrepresented by the Secretary of the Army Filed Oct. 28, 1963, Ser. No.319,625 7 Claims. (Cl. 333-) This invention relates to directionalcouplers and more particularly to a compact 3 db directional coupler ofthe waveguide type.

A waveguide directional coupler comprises a pair of waveguides whichshare a common wall. Energy is transferred between the two waveguides bymeans of one or more apertures in the common wall. A simple method ofobtaining directional coupling of two such waveguides consists of twoholes in the common wall spaced along the longitudinal axis by adistance equal to one quarter of the guide wavelength. When power flowsthrough one of the guides in a given direction, part of the microwavepower is coupled through each of the holes, exciting waves in the secondwaveguide which propagate in both directions from each hole. The wavesfrom each hole travelling in the same direction as the energy in thefirst waveguide reinforce each other while the backward waves canceleach other. The first waveguide is called the main guide and the secondthe auxiliary guide. The performance of a directional coupler ismeasured by its directivity and coupling, the directivity being theratio of power flowing in the auxiliary guide in the desired forwarddirection to that flowing in the backward direction and the couplingbeing the ratio of power input to the main guide to power output of theauxiliary guide. The directivity of a two-hole coupler falls off rapidlyas the operating frequency depart from the design frequency. Widebandoperation as well as increased coupling can be achieved by utilizing oneor more arrays of holes in which the coupling through the various holesof the array is made proportional to the coefiicients of a binomialexpansion. In a so-called Tchebycheif array, the coupling of thesuccessive holes is made such that the amplitude of the backward wave inthe auxiliary waveguide varies as a function of frequency as does theTchebycheif polynomial of order one less than the number of couplingholes. Multiple Tcheby-cheff or binomial arrays may be overlapped withsome of the central holes forming parts of two or more arrays and hencebeing of larger size than the holes at either end.

The directional coupler of the present invention was designed inresponse to the need for an X-band directional coupler of minimumlength. In order to achieve the required compactness as well as acoupling ratio of 3 db, it was found necessary to utilize a relativelysmall number of large coupling holes. In order to achieve the requiredtight coupling with a minimum length coupler, it was necessary to usecoupling holes substantially larger than those of either a Tchebychefi?or binomial coupling array. However, it was found that these shortarrays of large coupling holes caused several undesired resonanceswithin the operating range of the waveguide system. These resonances canbe eliminated by reducing the height or b dimension of both of thewaveguides in the coupling region. In order to match the impedance ofthe reduced height section to the rest of the waveguide system, astepped impedance transformer is utilized. A further size reduction isaccomplished by incorporating this impedance transformer within thecoupling region.

It is therefore an object of this invention to provide a compact 3 dbdirectional coupler.

It is a further object of the present invention to provide a novel anduseful X-band 3 db directional coupler.

3,230,482 Patented Jan. 18, 1966 Other objects and advantages willbecome apparent from the following detailed description and drawing, inwhich:

FIG. 1 is an isometric view of the one embodiment of the invention, and

FIGS. 2 and 3 are end and side views thereof respectively.

FIG. 4 is a plan view of the common wall of the two waveguides.

In FIG. 1 the directional coupler 5 comprises a pair of waveguides 8 and9 which share a common broad wall 10. The common Wall includes twolongitudinal arrays of relatively large coupling holes 12 and 14,arranged on opposite sides of the longitudinal axis of the common wall.End flanges 6 and 7 are provided for inserting the coupler into awaveguide system. The illustrative directional coupler was designed foruse in a microwave system comprising rectangular waveguides having a andb dimensions of .9 and .4 inch respectively and operating over thefrequency range of 8.2 to 12.4 kmc. In accordance with the invention,the height or b dimension of each of the waveguides comprising thecoupler is reduce-d from that of the rest of the waveguide system. Thisheight reduction was found to move the undesired resonances caused bythe relatively large coupling holes upward in frequency beyond theoperating range of the system. Referring to FIG. 3, the b dimension ofthe flanges is made the same as the b dimen sion of the waveguidesystem, which is .4 inch in the illustrative embodiment. The heightreduction is accomplished by increasing the thickness of the top andbottom broad walls 24 and 25 of the coupler in three steps 20, 21 and 22in the top wall and 11, 15, and 16 in the bottom wall. The totalreduction in height amounts to .065 inch in each of the waveguides 8 and9. This amounts to a reduction of approximately 16% in the b dimension.Each series of steps constitutes a Tche'by-cheif stepped impedancetransformer, so-called because the Tche'bycheff polynomial is used inits design. Such a transformer matches the impedance of the reducedheight coupler to the rest of the system with a minimum number of stepsand minimum standing wave ratio. The design of such transformers isdescribed in an article authored by the present inventor on pp. 1621 ofIRE Transactions on Microwave Theory and Techniques, vol. MTT3, #3,April 1955. The steps are spaced .38 inch apart, which is approximatelyone quarter of the waveguide wavelength at the center of the operatingfrequency range in the illustrative embodiment. In accordance with thedesign principles given in the cited article, the step heights are .020,.027 and .018 inch from left to right in FIG. 3 for both of theillustrated series of steps. Similar impedance transformers (no-t shown)are formed in the right hand ends of the top and bottom broad walls.

FIG. 4 is a plan view of the common wall of the directional coupler ofFIGS. 13. The common wall 10 contains two similar longitudinal arrays ofcoupling holes 12 and 14. Each array consists of six generallyrectangular holes 29 with longitudinal dimensions of .360 inch and atransverse dimension of .320". The six large holes of each array aresymmetrically disposed on either side of the transverse center line 2727of the common wall and the centers of the six holes are approximatelyonefifth of the a dimension or width of the common wall from eitheredge. The penultimate holes 30 at each end of each array are slightlysmaller than the six largest holes, being .335 inch along thelongitudinal direction of common wall 10 and .305 inch in the transversedirection thereof. Each of the four end holes 31 of each array are .264inch by .228 inch with the long dimension parallel to the long dimensionof the common wall 10 as is the case with all of the coupling holes. Thehole centers of each array are spaced by approximately one quarter ofthe guide wavelength at the center of the operating frequency range.This leaves only a space of approximately .020 inch between the sixlarge coupling holes 29 and approximately the same space between holes29 and 30, but a space of about .070 inch between the end holes 31 andthe penultimate holes 30. The entire coupler measures approximately fourinches in length.

While the invention has been described in connection with a specificembodiment, the inventive concepts disclosed herein are of generalapplication and hence the invention should be limited only by the scopeof the appended claims.

What is claimed is:

1. A compact 3 db directional coupled comprising, a pair of waveguideshaving a common broad Wall, two similar arrays of relatively largecoupling holes extending along the longitudinal dimension of said commonwall on either side of the longitudinal center line thereof, a flange ateither end of said pair of waveguides, said flange being adapted toconnect said coupler to a waveguide system of dimensions a and b, meansto reduce the b dimension of each of the waveguides comprising saidcoupler by increasing the thickness of the top and bottom broad Wallsthereof in step fashion, said steps comprising a Tchebychefi impedancetransformer.

2. A compact 3 db directional coupler comprising, a pair of Waveguideshaving a common broad wall, two similar arrays of relatively largecoupling holes extending along the longitudinal dimension of said commonwall on either side of the longitudinal center line thereof, means toconnect said coupler to a waveguide system of dimensions a and b where ais the interior width of said common Wall, means to reduce the bdimension of both of said waveguides relative to the b dimension of saidwaveguide system by increasing the thickness of the top and bottom broadwalls of both of said waveguides by means of a series of steps, saidseries of steps comprising a Tchebycheff impedance tranformer.

3. A compact 3 db directional coupler comprising a pair of X-bandwaveguides arranged to share a common broad wall, a relatively shortarray of large coupling holes extending along said common wall on eitherside of the longitudinal center line thereof, means to connect saidcoupler to an X-band waveguide system with interior dimension of a andb, means to reduce the height or b dimension of each of said waveguidesby approximately 16% relative to the b dimension of said waveguidesystem, said last-named means comprising a Tchebycheff stepped impedancetransformer formed in the top and bottom broad walls of each of saidwaveguides.

4. A compact 3 db directional coupler comprising, a pair of waveguideshaving a common broad wall, two similar arrays of coupling holesextending along the longitudinal dimension of said common wall on eitherside of the longitudinal center line thereof, said coupling holes beingsubstantially larger than those of either a Tchebychelf or binomialarray of equal coupling, means to connect said coupler to a waveguidesystem, the a dimension of said waveguide system being the same as thatof said waveguides and the b dimension of said Waveguides being lessthan that of said waveguide system, and means within said coupler tomatch the impedance of said coupler to the impedance of said waveguidesystem.

5. A compact X-band directional coupler comprising, a pair of waveguideshaving a common wall, two similar arrays of generally rectangularcoupling holes extending along the longitudinal dimension of said commonwall on either side of the longitudinal center line thereof, saidcoupling holes being substantially larger than those of either aTchebycheff or binomial array of equal coupling, means. to connect saidcoupler to a waveguide system, the a dimension of said system being thesame as that of said coupler, the b dimension of said coupler beingapproximately 16% less than that of said waveguide system, and aTchebychetf stepped transformer at either end of both of saidwaveguides.

6. A compact directional coupler for use in a waveguide system having afrequency range of 8.2 to 12.4 kilomegacycles comprising, a pair ofwaveguides having a common broad Wall, two similar arrays of relativelylarge coupling holes extending along the longitudinal dimension of saidcommon wall, said waveguide system having a ance of said coupler to thatof said waveguide system.

7. A compact directional coupler for use in a waveguide system having afrequency range of 8.2 to 12.4 kilomegacycles comprising, a pair ofwaveguides approximately four inches in length having a common broadwall, two similar arrays of coupling holes extending parallel to and oneither side of the longitudinal center line of said broad wall, saidcoupling holes being substantially larger than those of either aTchebychelf or binomial array of equal coupling, said waveguide systemhaving a and b dimensions of .9 and .4 inch respectively, saidwaveguides having an a dimension the same as that of said waveguidesystem and a b dimension reduced approximately 16% from that of saidwaveguide system, the reduction in said b dimension being accomplishedby a series of steps in the thickness of the top and bottom walls ateither end of said waveguides, said series of steps comprising aTchebychetf impedance transformer.

References Cited by the Examiner UNITED STATES PATENTS 12/1957 Adcock etal. 33310 l/1958 Sferrazza 33-10 OTHER REFERENCES HERMAN KARL SAALBACH,Primary Examiner.

G. TABAK, Assistant Examiner.

1. A COMPACT 3 DB DIRECTIONAL COUPLED COMPRISING, A PAIR OF WAVEGIUDESHAVING A COMMON BROAD WALL, TWO SIMILAR ARRAYS OF RELATIVELY LARGECOUPLING HOLES EXTENDING ALONG THE LONGITUDINAL DIMENSION OF SAID COMMONWALL ON EITHER SIDE OF THE LONGITUDINAL CENTER LINE THEREOF, A FLANGE ATEITHER END OF SAID PAIR OF WAVEGUIDES, SAID FLANGE BEING ADAPTED TOCONNECT SAID COUPLER TO A WAVEGUIDE SYSTEM OF DIMENSIONS A AND B, MEANSTO REDUCE THE B DIMENSION OF EACH OF THE WAVEGUIDES COMPRISING SAIDCOUPLER BY INCREASING THE THICKNESS OF THE TOP AND BOTTOM BROAD WALLSTHEREOF IN STEP FASHION, SAID STEPS COMPRISING A TCHEBYCHEFF IMPEDANCETRANSFORMER.